1. Field of the Invention
The present invention relates generally to satellites and, more particularly, to antenna pointing and to wide field-of-view attitude acquisition of satellites.
2. Description of the Related Art
The diagram 20 of FIG. 1A illustrates a satellite 22 that orbits in an orbital plane 24 about the earth 26. The satellite has a satellite body 28 which carries an antenna system 29 and solar panels 30 that generate power for the satellite. Although the satellite's orbital plane 24 may be coplanar with the earth's equatorial plane 32, it is shown more generally as having an inclination 34.
The satellite 22 may be in a synchronous orbit or alternatively, in a nonsynchronous orbit. FIG. 1A illustrates the synchronous alternative by showing the satellite in positions 22A, 22B and 22C at exemplary times To, To+6 hours and To+12 hours. The satellite 22 provides a service (e.g., communication service) to a service area 40 on the earth which is shown in corresponding positions 40A, 40B and 40C.
FIG. 1A also illustrates the nonsynchronous alternative by indicating that the satellites at positions 22B and 22C may be different satellites 36 and 38. In the nonsynchronous alternative, FIG. 1A represents one instant in time (e.g., the time To) and the satellites 22, 36 and 38 serve respective service areas 40A, 40B and 40C.
FIG. 1B is an enlarged view of the satellite in position 22A. This figure shows that the antenna system 29 of the satellite 22 generates a payload beam 42 which forms a payload footprint 44 on the earth 26. The payload beam generally includes a large number of individual spot beams. In order to enhance the satellite's provided service and reduce the energy needed to provide that service, the payload footprint 44 is preferably coincident with its respective service area. Stated differently, it is important to reduce service error which is any difference between the payload footprint 44 and its respective service area.
The importance of reduced service error has created a need for satellite methods and structures that improve antenna pointing. Sources of error in antenna pointing include mechanical misalignment, thermal deformation, ephemeris error and orbit error. Most systems that improve antenna pointing depend upon sensed signals from attitude references (e.g., sun, stars and earth's horizon). A conventional attitude reference that improves antenna pointing is a beacon ground terminal that radiates a beacon signal. This provides the satellite's receiving antennas with a reference signal from a predetermined terminal location. Beacon systems, however, require additional satellite hardware and the cost of a dedicated beacon terminal.
Accordingly, various alternatives have been proposed. For example, U.S. Pat. No. 3,060,425 radiated a suppressed-carrier, double sideband signal from a plurality of antennas that were arranged transversely to a selected axis of a satellite. These signals were received at an earth-based terminal and demodulated to yield phases and amplitudes indicative of the satellite's attitude with respect to the selected axis. This method requires accurate interferometry equipment and is difficult to implement with conventional communication terminals.
In a method of U.S. Pat. No. 4,790,071, three different phase-shifted pulses, one sum pulse and two delta pulses, are generated on a satellite and transmitted to an earth-based terminal. The delta pulses and sum pulse are used to form two delta-to-sum ratios that indicate relative attitude between the satellite and the terminal. This method requires special phase shift patterns of the antenna which can not be used to generate regular service beams.
U.S. Pat. Nos. 4,599,619 and 4,630,058 apply two satellite-generated beacon beams, one regular beam from the satellite communication antenna and one broad beam from a separate antenna that covers a region including and greater than that covered by the regular beam. The beacon beams are received at ground terminals that are positioned near the periphery of the regular beam. Ratios of the regular beam to the broad beam are thereby produced and are used to determine pointing errors of the communication antenna. To practice this method, the satellite must carry the additional antenna and a large number of ground terminals must be appropriately positioned.
A method of U.S. Pat. Nos. 5,697,050 and 5,758,260 is directed to a satellite whose antenna generates a moving beam pattern on the earth's surface wherein the beam pattern comprises a plurality of sub-beams. A signal radiated from at least one ground-based transmitter terminal is received with the satellite's antenna and that received signal is retransmitted to the ground terminal. The gain of the received signal is determined at the ground terminal and compared to an expected gain to derive antenna pointing correction signals. This method is restricted to pointing of satellite receiving antennas that have moving beam patterns on the earth's surface.
U.S. Pat. No. 5,812,084 configures a satellite's phased-array antenna in a “straight-through” mode in which all radiating elements radiate with the same amplitude and phase. The antenna's attitude is then estimated based upon straight-through gains measured at two or more receiver sites. Most satellite service beams are, however, not generated in such a “straight-through” mode.
U.S. Pat. No. 4,910,524 oscillates the pointing direction of a satellite transmit beam to produce a periodic or repetitive displacement of a ground pattern, and measuring the resultant oscillatory variation in flux density at a ground station or ground stations to determine the antenna beam pointing errors. For most satellites, however, the addition of a deliberate oscillation of the payload would be an added burden on the satellite, and it is itself another source of antenna pointing error.
U.S. Pat. No. 6,150,977 measures the signal strength of a first spot beam at at least three unique locations on the ground to determine at least one attitude component of the antenna pointing error of a satellite antenna. The requirement that at least three unique ground measurement locations be provided for a single beam is unnecessarily restrictive.
The paper by Loh, “On Antenna Pointing for Communications Satellite” discusses many methods of determining satellite antenna beam pointing, including sun, earth, star and beacon sensors. There is also discussed a system of pointing based on a on-board multiple-beam-antenna (MBA) system. The MBA sensing system processes the magnitudes of signals received from a known uplink site by singlet beams of an on-board MBA system to provide the error for antenna pointing control. Providing good attitude information using this single uplink site taught by Loh requires that position of the uplink site in the singlet beam pattern provides good observability of attitude. Most combinations of uplink site and singlet beam pattern optimized for communication will not have good observability. The current invention addresses this by using multiple uplink sites.
Loh describes three techniques of closed loop control of antenna beam pointing classified under “A.2 On-Ground Sensors”. These are “Ratio of Signals at Various Sites”, “Downlink C/KT's measured by a Spectrum Analyzer” and “Location Determination Using Singlets of MBA”. However, the first technique simply describes and references the teachings of U.S. Pat. No. 4,630,058, discussed above. The second technique “assumes that the downlink C/KT's of each FDMA transponder is measured at a ground station by a spectrum analyzer. The ratios of measured C/KT's to the desired values are used as pointing error for footprint control.”
A system based on this is described in the section “Ground-based Closed-loop Satellite Antenna pointing Control System”, and depicted in FIG. 10 (using four ground sites for one antenna beam), of the Loh paper. Here Loh is teaching a system very similar to that of U.S. Pat. No. 6,150,977, and teaches away from systems using multiple ground sites and multiple antenna beams.
The third technique is to receive the signals from a known uplink site by singlet beams of an on-board multiple-beam antenna (MBA) and to transpond these signals to the ground, where beam pattern databases and processing software reside in a computer on the ground processing center. As in the other MBA system Loh describes, most MBA singlet patterns would have to be modified to provide good observability using a single uplink site.
It is therefore apparent that conventional antenna pointing methods have generally required the addition of substantial processes and structures beyond those required to realize the intended services of satellites or their application has been limited to antennas that generate moving patterns on the earth's surface.
With respect to satellite attitude acquisition, conventional beacon-based satellite attitude acquisition methods have typically been restricted to narrow fields-of-view because they utilize ground-based beacon signals.